1. Field of the Invention
The present invention relates to discriminators and, more specifically, to a discriminator automatically generating optimum thresholds and discriminating the amplitude of an input multi-level signal with the generated thresholds.
2. Description of the Background Art
Conventionally, a multi-level signal varying in amplitude among three or more values can be sent and received in a transmission system. In such multi-level signal, information is assigned to each amplitude value. The receiving side in the transmission system discriminates the amplitude of the received multi-level signal using predetermined thresholds. However, with fluctuations in amplitude of the multi-level signal due to various factors, fixed thresholds are not preferable because each of the thresholds may be shifted from a mid-level between one amplitude value and the amplitude value just one level below or above. As a result, the receiving side may perform erroneous amplitude discrimination, or the resultant pulse width after discrimination may be distorted.
Furthermore, in a transmission system of a binary signal indicative of "Hi" or "Lo", the receiving side may perform amplitude discrimination with a discriminator CD as shown in FIG. 12. In FIG. 12, the discriminator CD is adapted to automatically generate a threshold that is appropriate for amplitude discrimination (so-called automatic threshold control) even if the amplitude of the received binary signal may fluctuate. The discriminator CD includes an input terminal 21, a branching part 22, a peak detector 23, a base level generator 24, a threshold generator 25, and a comparator 26.
The branching part 22 branches a binary signal fed through the input terminal 21 into two, outputting one to the peak detector 23 and the other to the comparator 26. The peak detector 23 detects and holds a peak value of the received binary signal. The detected peak value is equal in electric potential to "Hi" of the binary signal which may fluctuate, and fed to the threshold generator 25. The base level generator 24 generates a base level equal in electric potential to "Lo" of the binary signal, and outputs the base level to the threshold generator 25. The threshold generator 25 generates a threshold having a mid-level between the received peak level and base level, and outputs the threshold to the comparator 26. The comparator 26 compares the amplitude of the branched binary signal with the threshold, and outputs the comparison result. As such, the peak detector 23 detects the peak value of the binary signal, enabling the threshold generator 25 to automatically generate the threshold even with fluctuations in amplitude of the binary signal.
For applying the above discriminator CD to multi-level signal transmission systems, the threshold generator 25, which generates the mid-level value, is modified to generate a plurality of thresholds each having a mid-level between one amplitude value and the amplitude value just one level below or above of a multi-level signal.
In general, a marking ratio of appearing each amplitude value of a multi-level signal is lower than that of a binary signal. Therefore, the maximum amplitude value of the multi-level signal does not necessarily appear during an interval defined by a time constant of the peak detector 23. In such case, the peak value detected by the peak detector 23 does not become equal in electric potential to the maximum amplitude value of the multi-level signal. As a result, the threshold generator 25 cannot generate accurate thresholds.
Furthermore, the marking ratio in the multi-level signal is difficult to manage compared with that in the binary signal. In addition, the mean value of the multi-level signal does not necessarily indicate the mid-level of the amplitude. Therefore, replacing the peak detector 23 with a mean value detector is not practical.
Due to the above background, in conventional multi-level signal transmission systems, the transmitting side is required to transmit at least a maximum amplitude value of the multi-level signal (and a minimum amplitude value thereof as required) to the receiving side every predetermined time to charge a capacitance of the peak detector 23. Or, the transmitting side is required to produce a multi-level signal in which at least its maximum amplitude value (and also minimum amplitude value, in some cases) moderately appears. Therefore, when the conventional discriminator CD is applied to a multi-level signal transmission system, the transmitting side is disadvantageously under restrictions on generating a multi-level signal.